fix(bazel): alternate method of installing bazelisk in docker image

[justbuchanan]

Before this change, attempts to build the bazel extractor image fail with:

bazel build //kythe/extractors/bazel:docker

ERROR: error loading package '@@com_github_bazelbuild_bazelisk//': Unable to find package for @@build_bazel_rules_nodejs//:index.bzl: The repository '@@build_bazel_rules_nodejs' could not be resolved: Repository '@@build_bazel_rules_nodejs' is not defined.
ERROR: /usr/local/google/home/justinbuchanan/src/github/kythe/kythe/extractors/bazel/BUILD:3:13: error loading package '@@com_github_bazelbuild_bazelisk//': Unable to find package for @@build_bazel_rules_nodejs//:index.bzl: The repository '@@build_bazel_rules_nodejs' could not be resolved: Repository '@@build_bazel_rules_nodejs' is not defined. and referenced by '//kythe/extractors/bazel:docker'
ERROR: Analysis of target '//kythe/extractors/bazel:docker' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.330s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

I copied the bazelisk download/install snippet from our presubmit-runner image.